A:   My thoughts on this would be to use a non-selective herbicide such as glyphosate. This is assuming that the weeds are so bad that salvaging any oat hay from the field is not possible; or maybe the oats have already been grazed some. If there is the possibility or plans to hay the oats later in June; then using 2,4-D would be recommended. Mowing will provide some control to the weeds but there will be some regrowth that comes back and one would be faced with a similar problem 2 to 3 weeks after mowing. For the purpose of fallowing, total kill with a herbicide will leave residue in the field and prevent soil moisture use by the weeds.